1. Field of the Invention
This invention relates to a digital-to-analog converter, and in particular to a digital-to-analog converter that attempts to reduce an amount of noise present in an output signal, and to a method of operation of a digital-to-analog converter.
2. Description of the Related Art
In many electronics devices, digital signals are used in order to allow signal processing operations to be performed, or to allow data to be stored in a convenient form. However, it is often necessary to use analog signals, for example to drive devices such as audio equipment. In such situations, and many others, digital-to-analog converters are used to convert a digital input signal to an analog output signal.
Digital-to-analog converters are known, in which a value of a digital input signal is used to select a number of single-bit digital-to-analog converter elements. The outputs of these single-bit digital-to-analog converter elements are then summed together, in order to produce an analog output signal.
U.S. Pat. No. 6,583,742 discloses a digital-to-analog converter, comprising:                a plurality of pairs of digital-to-analog converter elements; and        an adder, connected to form an analog output signal as the sum of the outputs of the pairs of digital-to-analog converter elements; and further comprising:        an element matching circuit, connected to receive a digital input signal, and apply respective inputs to the pairs of digital-to-analog converter elements, wherein the element matching circuit comprises an element matching stage associated with each of the pairs of digital-to-analog converter elements, and wherein each element matching stage comprises:        an input for a respective stage remainder value, the remainder value having a parity determined by a value of a least significant bit thereof;        first and second quantizers, for forming a pair of quantizer outputs, a sum of said quantizer outputs being constrained to be equal to the parity of the remainder value; and        first and second feedback loops, associated with the first and second quantizers respectively, for forming respective first and second loop values and applying said loop values as inputs to the first and second quantizers respectively.        
In this prior art device, a small amount of random noise is added to the inputs of the first and second quantizers, in order to randomize the decisions taken by the quantizers, and thereby reduce the likelihood of repetitive patterns being generated in the output signals.
It is noted that the effect of this random noise in altering the state of the quantizers will be much greater when the absolute values of the inputs of the first and second quantizers are substantially equal.